This invention is related to photoluminescent, pressure sensitive paints, and, in particular, to such paints suitable for indicating pressure changes at the surface of an aerodynamic structure during wind-tunnel testing.
Aerodynamic structures such as aircraft and aircraft components are commonly tested in a wind tunnel to gather data for use in verification of characteristics and in design improvements. Various quantities are measured in the wind-tunnel testing, including, for example, the pressure distribution at the surface of the structure. The pressure information is used to calculate air flows and force/pressure distributions over the structure.
A number of techniques have been employed over the years to make the pressure measurements. Mechanical or electronic pressure sensors can be affixed to the external surface of the structure. Such sensors ordinarily have a portion extending above the surface into the airstream, which itself can alter the airstream and the measured values. The sensors ordinarily are relatively large in size, which limits the spatial resolution of the data that is gathered and also limits the number of sensors that can be employed. In another approach, small orifices are provided in the surface to act as pressure taps. The taps communicate at one end with the pressure at the surface of the structure and at the other end with a pressure transducer. If the structure under test is a subscale model, it is ordinarily quite difficult to use a large number of pressure taps due to the size of each tap and its pressure transducer. The spatial resolution of the measurements and the number of sensors that can be used is therefore limited. Lastly, models utilizing pressure taps are time consuming and expensive to build.
A more recently developed alternative approach is luminescence barometry, described, for example, in U.S. Pat. No. 5,359,887. In this technique, the surface of an aerodynamic structure is coated with a paint constituting a formulation of a binder and an active agent that emits light when excited by radiation of a particular type, such as ultraviolet or visible blue light. For some types of active agents, the presence of oxygen, such as found in the air, quenches or reduces the light emission. The extent of quenching is proportional to the partial pressure of the oxygen, or, stated conversely, the light output of the active agent is inversely proportional to the partial pressure of the oxygen. The binder is selected to hold the paint in place on the surface of the aerodynamic structure, yet permit the oxygen in the atmosphere to permeate therethrough and reach the active agent to perform the quenching function.
The higher the pressure of oxygen in the atmosphere contacting the luminescent paint, the lower the light emission of the paint. The intensity of light emission of the paint is therefore a useful measure of the local oxygen partial pressure, and thence the total local air pressure, of the atmosphere in contact with the paint. The light intensity is measured optically, so that the limit of spatial resolution is typically determined by the spatial resolution of the optical system. Thus, the aerodynamic structure is effectively "instrumented" for wind-tunnel pressure testing by painting the structure with the pressure-sensitive paint, illuminating the structure with the required wavelength of radiation, and measuring the luminescence and light output intensities over the surface of the model using an optical imaging system.
Although the existing paints are operable, there is an ongoing need for a paint formulation that is more sensitive to pressure change, is durable, and is easily applied to produce a smooth surface on the aerodynamic structure. The present invention fulfills this need, and further provides related advantages.